Source:Great Muslim Quotes
And of the diseases that afflict dua and prevent its response is that a person is hasty in expecting a response, and the response is delayed, so he gives up hope, and leaves du’a. His example is like that of a person who planted a seed, or a seedling, then guarded it, and took care of it, and watered it, but when it delayed in (giving its fruit) and reaching perfection, he left it did not take care of it anymore!
(Ibn al-Qayyim)